Solar Technology for Boats

Solar energy for boats

- Green energy on board - 

Many boat owners use solar modules. Sometimes only one, sometimes several. Modules can be mounted directly on the deck, on a stable equipment rack or they can be used as mobile units and, if required, placed in a free place on deck or on the bimini in the sun. They are also used in the caravaning industry, where the sun's energy is harnassed to provide power for a number of appliances.

Where are solar power systems used?

Solar power systems are also known as photovoltaic or PV systems/generators. They are not only used to power boats or mobile homes, but also for holiday homes, garden sheds and mountain huts, i.e. in all places where there is limited or no access to a public power supply. The solar systems used in this way generate a 12 or 24 volt DC current and are known as island or off-grid systems.

What are the components or a solar power system for boats?

A solar system consists of one or more solar modules, a charge regulator, electrical installation material such as cables for connecting to the module and battery, plug connections, fuse holders / fuses and assembly material such as module holder, screws, adhesive, cable bushings, etc.

How does a solar system for boats work?

The solar module converts sunlight into electrical energy, direct voltage and direct current (DC). A charge regulator limits the rate at which voltages and currents generated by the solar module are added or drawn from a battery (bank) and supplied to appliances. An on-board battery stores the electrical energy so that it can also be used when no solar energy is available, e.g. at night. The DC appliances are either supplied by the solar power system directly or the battery. If required, an inverter can be used to generate an alternating current (AC) from the battery and used to operate 230 V appliances.

How are solar power systems intalled on board? 

Installing a solar system on a boat is a simple DIY job. In just three steps you will have an additional energy source that automatically converts the suns energy into power that can be stored in the battery or used in your boat's electrical system, without any further action on your part.

1.
Choose a place that is not in the shade and fix (glue or screw) the solar module in the desired position.

2.
Mount the charge regulator, connect the PLUS and MINUS cables to the regulator output (battery symbol) and to the battery. Information on the required cable cross-section can be found in the assembly/operating instructions of most manufacturers. Configure the regulator to the battery type and do not forget to install a fuse close to the battery in the PLUS line.

3.
Route the PLUS and MINUS wires from the module to the charge regulator input (module symbol). Covering the module while you are setting up the connection will avoid sparking on plug connections or on the terminals of the regulator.

Do not underestimate the importance of this third point. It could be the only difficult part of the whole installation project and so is worth dedicating more time and attention to it. This is because the wires coming from the solar module must be properly sealed from the outside and routed to the regulator below deck with sufficient strain relief. Sometimes tight spaces or places that are difficult to access are a particular challenge here.

Our tip: While looking for a suitable place to install the module, you should also think about how the connecting cables will be routed!! Many modules are supplied with connection cables of sufficient lengths. If cable lengths are not long enough or if several modules are to be connected in parallel, care must be taken to ensure sufficiently large cable cross-sections. The aim is to keep voltage loss from the cable to the regulator as low as possible, ideally below 2% of the module voltage.

What is important when planning the installation of solar power systems?

It is important to know the specifications and operating conditions of the solar modules and charge regulators and to take these into consideration during system planning and installation. The aim is to ensure optimum lighting conditions for the solar modules throughout the day. Sunlight should ideally hit the module directly, vertically and without any shading. Due to the way the sun moves during the day, the output power of a PV system is not constant. It is highest at midday (when the sun has reached its zenith). Solar modules can still generate energy even when the sun is low in the morning and evening hours and in low light, such as when the sky is overcast. In this case, module output voltage is only marginally dependent on light exposure, while the output current is proportional to exposure intensity. (Maximum irradiance = maximum power, half irradiance = 50% power, etc.)

If you plan to use PV throughout the year, bear in mind that the angle of light irradiation changes not only during the course of a day, but also from season to season. In summer there are more hours of sunshine per day than in winter when the sun is low.

What solar output do I need?

The answer to this question is not easy. It depends on what you expect from your solar system:

1. Do you want cost free solar energy to reduce the drain on your consumer battery during the course of a day and provide just enough power for a cool box or refrigerator, for example?

Step Do you want to charge your batteries with solar energy if there is no shore connection available?

Step Are you just looking for a way to maintain the charge level of your batteries?

4. Do you want solar energy to give you self-sufficient power for 24 hours or longer? Then the solar power system must be able to generate enough power for all appliances in your on-board network and also charge the consumer battery.

To be able to answer this question, you need to know the energy requirements of your appliances and how long they will operate. This means you need to know how many ampere hours (Ah) or watt hours (Wh) the appliance or appliances require over a defined period of time. There are several ways to determine your individual energy requirements:

1. Create an energy plan
2. Determine requirements with the help of a battery monitor
3. Search for reliable information on the internet

Where should I install a solar system on board? 

Solar modules should be exposed to direct sunlight for as long as possible. The maximum output is generated when the sunlight hits the module vertically. Anyone who has already done some work on the subject of solar energy knows that modules with higher output are also larger in size. Example: For 50 watts you need an installation area of approx. 0.4 m², for 100 W you need approx. 0.7 m². Ultimately, however, the size of the module (length and width) will determine whether one or several of them will fit onto the intended installation space on board, which in turn will determine how much solar output can be generated. If you do not have a large enough surface area to install a solar module securely, you should consider using mobile modules with textile backing. You can place them anywhere, stand them up, strap them on to something or, if the modules have TENAX or LOXX snap fasteners, you can easily attach them securely to the bimini or other textile surfaces.

What specifications must a solar module for boats have?

Frame modules are often mounted where the height of the module frame or mounting material does not compromise the use of the boat or vehicle and where the module is sufficiently ventilated, e.g. on an equipment rack or on large and open roof surfaces. Semiflexible modules are perfect for mounting directly on deck or on slightly curved surfaces. In some cases, the installation location may also require the module to be walkable. You should also consider the module junction box and connection cables.

How and where can I fit the connection cables for solar modules and how do I get the cables below deck?

If you want to connect several modules in parallel, this can be done on deck. The only exception are modules with a separate cell protector. These can only be connected in parallel on the output side of the cell protectors.
It is of course also possible to run the connecting cables of each module to the charge regulator and connect them in parallel there. Laying a single multicore module connection cable is easier than laying two individual PLUS and MINUS cables.

If you want to use mobile modules on board and don't want to have long cables lying around which could be a trip hazard, you could install sockets in suitable places. The sockets could also be used as a deck connection point for solar cables to the charge regulator. You can connect several sockets in parallel (connect PLUS with PLUS and MINUS with MINUS). Maybe you want to experiment with one module first and expand the system later. You can do this, but try to think about what you want to do now so that you don't have to start all over again later.

What types of solar panels are available for boats?

- Overview -

Cell technology

A solar module or photovoltaic module (PV module) converts sunlight into electrical energy. Wherever large mounting surfaces are not available for a PV system, there is a preference for modules with a high degree of efficiency. In most cases these are modules with mono- or polycrystalline cells. Monocrystalline modules now reach efficiencies of over 20%. Modules with polycrystalline cells tend to be slightly lower with up to 20%. Other cell technologies (thin-film, CIS/CIGS) are less efficient. Nevertheless, these technologies are worthy of mention, because they have advantages over mono- and polycrystalline cells, such as low weight, less loss in scattered light or partial shading, and in high temperatures. However, to achieve the same performance as mono- or polycrystalline modules, much larger mounting surfaces are required.

Module Efficiency

Module efficiency indicates the percentage of solar irradiation a solar module can convert into electricity. It is not the same as the efficiency of the solar cells used in the module, because the size and overall construction also influence the efficiency of the module as a whole. The basic element of most modules is the photovoltaic cell, which usually consists of the semiconductor material, silicon. Due to the different ways in which silicon is processed, a distinction is made between monocrystalline and polycrystalline cells.

Monocrystalline cells

High-purity molten silicon is required for the production of monocrystalline cells. Bar-shaped single crystals are extracted from the silicon melt and then cut into wafers. The monocrystalline cells produced in this way guarantee high efficiencies of over 20%. This means that they are able to convert about 1/5 of solar irradiation into electrical energy

Monocrystalline back-contact cells

Continuous further development of this cell type has enabled it to achieve efficiencies of over 25%. Monocrystalline back-contact cells are contacted on one side and can be identified by there being no silver-coloured contacts on their very dark blue top side. This means that a larger free surface area is exposed to sunlight, which makes the cells more efficient.

Polycrystalline cells

The manufacturing process of polycrystalline, often also called multicrystalline cells, is simpler. Here liquid silicon is poured into blocks and the blocks are then cut into wafers. When the material solidifies, it does not form a single crystal, but many different crystal structures. Polycrystalline modules usually give off a bluish shimmer. Due to the uneven crystal structure, light hitting it is reflected differently, causing the cell surface to twinkle. Polycrystalline cells are cheaper to produce, but have a slightly lower efficiency than monocrystalline cells.

Thin-film cells

For the production of thin-film cells, amorphous silicon is deposited very thinly on a glass plate or film. Amorphous silicon cells have a lower efficiency than crystalline cells, but they are less sensitive to temperature changes and are particularly suitable for use under difficult lighting conditions, such as partial shading or when light is not homogeneous (scattered light).

CIS/CIGS cells

These cells are not made of silicon and their name refers to the chemical elements used: copper, indium, gallium and selenium. The most frequently used combinations are copper indium disulphide CuInS2 and copper indium/gallium diselenide Cu(In,Ga)Se2. As with thin-film cells, the CIS/CIGS absorber material is applied to glass or plastic surfaces. Because they have the highest spectral sensitivity of all thin-film cells, CIS/CIGS cells are highly efficient. The technology is generally used for flexible solar modules. 

Solar module structure

Most solar modules consist of a certain number of single silicon-based photovoltaic cells. A single solar cell usually generates an electrical voltage of 0.5 V to 0.6 V. This is far too little to charge a 12 V battery, for example, which would require a voltage of more than 15 volts. To achieve a higher voltage, multiple cells are fixed on a backplate and connected in series until the required voltage is reached. Flat metal foils inside the module are used to connect the cells to the module terminals. In doing so, mono- and polycrystalline cells have a lower output voltage at high temperatures.
Example: A 12 V module consists of 36 cells* connected in series. This results in an open circuit voltage (Uoc) of about 21V and the voltage at maximum power (Ump) is about 17 to 18V. A 24 V module consists of twice the number of cells connected in series, namely 72, and this results in double the voltage values.
* Depending on module design and cell type, a 12 V module can consist of a series connection of 32 to 40 cells. Solar cells or modules connected in series are known as strings.

How much electricity does a solar cell supply?

The current generated by the module depends primarily on the size of the solar cells. For example, a commercial solar cell produces a current of 30 to 36 mA per cm² under optimal operating conditions. A monocrystalline solar cell with an area of 15.6 x 15.6 cm (=243 cm²) would therefore deliver a current of between 7 and 9 A. With this knowledge, people are inclined to opt for modules with large cells. This may be a good idea for large solar systems whose modules are well exposed to the sun without much shading. Marine or caravan PV systems often consist of only one solar module. Since one cannot rule out the possibility that this module will be shaded during the course of the day, this can then lead to a significant loss of power. This is why some manufacturers have optimised their modules. Instead of one string with large cells, 2 or more strings with smaller cells are used and connected in parallel. Under good lighting conditions, these modules generate almost the same current as those using large cells, but even if individual cells or a cell string are shaded, they still continue to generate the same power as the remaining cell strings! 

What are bypass diodes and why is hotspot protection needed?

Let's recap: Solar cells are connected in series and parallel to a solar module. The number of cells connected in series gives the voltage of the module, while the number of cell strings connected in parallel determines the current.
If many cells are connected in series, if individual cells are shaded, the shaded cell or lamination material may overheat and destroy them, causing the module to bubble and burst in the shaded cell area. Such damage is known by experts as a hotspot. To avoid hotspots, bypass diodes are connected antiparallel to the solar cells.  In practice, a bypass diode is used for 15-20 cells connected in series. The anode of the diode is connected to the negative terminal of the cells to be protected and the cathode to the positive terminal. The diodes do not cause any losses because no current flows through them during normal operation. Bypass diodes also allow current to flow through the PV module when it is partially shaded.

Solar modules with aluminium frames

Here, the cells and cell carrier are mounted in an aluminium frame, covered with tempered glass. They have a waterproof back and are sealed all around. On the back of the module there is a waterproof junction box, which often also contains blocking or bypass diodes. Many modules are already equipped with two solar connection cables (PLUS and MINUS) and plug connections. The junction box should not be opened unless absolutely necessary or only after checking with the module manufacturer/supplier, as it must always be absolutely certain that it is reliably sealed.

Semiflexible solar modules

This type of module requires no frame or glass. Instead, the cells and their backing plate are placed in a high-quality ETFE and EVA foil laminate. The term semi-flexible indicates that these modules are not rigid like frame modules, but they can be firmly mounted (screwed and/or glued) on a curved surface. The widespread opinion that ALL semi-flexible modules are walkable or mobile is not true. Semiflexible modules are only walkable if the manufacturer specifically authorises this type of use. For this to be possible, modules must be firmly mounted and lie completely flat on the surface. There is a temptation to use semiflexible modules for mobile use, for example by placing them on a bimini. However, since powerful modules with large lengths would be constantly bent back and forth by the wind while on the bimini and when transported to the site, there is a risk that the internal cell connections could break over time, which could lead to the total failure of the module.

Our tip: If you want to have a semi-flexible module for mobile use, you should stiffen its back with suitable material. A light but stable 3 mm aluminium sandwich panel is suitable for this purpose. These plates can be found online and you can have them cut to size. With a little thought, a module on such a plate can be used in a variety of ways: The panel should be a few centimetres larger to make room for additional drill holes and fixing materials. This way the module could not only be installed on the bimini, but could also be attached to a railing or pushpit. With a bit of skill, you could even make it adjustable, so that you can adapt it to the position of the sun.